Gamb it woudld be interesting if Carlo and Elizabeth had a child named Phillip or Louis before they died because it means that Philip II holding would be divided into Spain, England-Burgandy, and Italian holdings. The Italian Holding would go to his 3rd son and might lead to an earlier Italian unification. Spain would go to his grandson and England-Burgandy would be inherited by Henry XI.
I don't quite buy this, sorry. From what I've studied, there was no belief there that Milan should be an independent entity. It was simply too valuable to Spain. It held open the Spanish Road - the military supply line from the Med, up through Milan and Genoa to Franche Comte and the Netherlands; it was an important bastion to ensure Spanish control of the North of Italy against the French and against any trouble-making Italians; it was an important economic centre and it was a prize possession. Any potential third son would be given titles and land, but no Milan. Maybe they would get the contemporary Duchy of Parma - a fief state carved out of a bit of Milan but very much considered part of the Hapsburg Empire and under Milanese suzerainty - but Milan was too important to hand away to another son, who's descendants would be all too likely to turn against their former family. An independent England could be trusted, largely as France was too much of England's enemy for England to be thought to side with it against Spain for long, and this was pretty much the case, but an independent Milan would be a second rate, even third rate power, and not even the uncontested strongest state in Italy. It would have to be expected that it would forsake ties to Spain to further its own gains, and France would be a sideshow for Milan, so the Spanish couldn't expect them to feel obliged to turn back to their kinsmen regularly. No, far more likely within a generation an independent Milan would start fighting Venice and Florence for dominance in Italy, and with the shifting alliances it's entirely feasible that they would side against a Spanish alliance to do so.
Also, the talk of Italian Unification is something of a modern-day phenomenon. Most people here when they talk about the Italian states seem to have an interest in shoe-horning in a Kingdom of Italy. This wouldn't happen. Italy showed no interest whatsoever in unification before the sudden rise of Italian nationalism around the early 19th century. In fact I'd go so far as to say than Italy was one of the most anti-unification regions around. When one state threatened to dominate Italy, the rest of them would group together to smack it down a level. This is the reason that the Italian Wars went on so long. Coalitions would happily form to oppose rising states, but all too often a successful defence would actually just cause the leader of the coalition to become the new threat, all of its allies would desert to the recently defeated country and the war would start again. The sancrosanctity of the Papal States' independence was a key motivator in keeping Italy dominance-free, too, and the Popes were all too happy to take a leading role in forming these Coalitions. A Hapsburg enthroned independently in Milan would only add to this, especially as in this era the Papacy was in a Franco-Hispanic struggle, with the College of Cardinals fighting over French and Spanish supremacy. With a Hapsburg sovereign in Milan, the tide could fell flow strongly towards the French.